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git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@323859 c90b9560-bf6c-de11-be94-00142212c4b1
318 lines
10 KiB
XML
318 lines
10 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<!-- $Revision$ -->
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<chapter xml:id="language.constants" xmlns="http://docbook.org/ns/docbook">
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<title>Constants</title>
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<simpara>
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A constant is an identifier (name) for a simple value. As the name
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suggests, that value cannot change during the execution of the
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script (except for <link linkend="language.constants.predefined">
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magic constants</link>, which aren't actually constants).
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A constant is case-sensitive by default. By convention, constant
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identifiers are always uppercase.
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</simpara>
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<para>
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The name of a constant follows the same rules as any label in PHP. A
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valid constant name starts with a letter or underscore, followed
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by any number of letters, numbers, or underscores. As a regular
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expression, it would be expressed thusly:
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<literal>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</literal>
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</para>
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&tip.userlandnaming;
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<para>
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<example>
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<title>Valid and invalid constant names</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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// Valid constant names
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define("FOO", "something");
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define("FOO2", "something else");
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define("FOO_BAR", "something more");
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// Invalid constant names
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define("2FOO", "something");
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// This is valid, but should be avoided:
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// PHP may one day provide a magical constant
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// that will break your script
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define("__FOO__", "something");
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<note>
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<simpara>
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For our purposes here, a letter is a-z, A-Z, and the ASCII
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characters from 127 through 255 (0x7f-0xff).
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</simpara>
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</note>
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<simpara>
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Like &link.superglobals;, the scope of a constant is global. You
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can access constants anywhere in your script without regard to scope.
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For more information on scope, read the manual section on
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<link linkend="language.variables.scope">variable scope</link>.
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</simpara>
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<sect1 xml:id="language.constants.syntax">
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<title>Syntax</title>
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<simpara>
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You can define a constant by using the
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<function>define</function>-function or by using the
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<literal>const</literal> keyword outside a class definition as
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of PHP 5.3.0. Once a constant is defined, it can never be
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changed or undefined.
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</simpara>
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<simpara>
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Only scalar data (<type>boolean</type>, <type>integer</type>,
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<type>float</type> and <type>string</type>) can be contained
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in constants. It is possible to define constants as a
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<type>resource</type>, but it should be avoided, as it can cause
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unexpected results.
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</simpara>
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<simpara>
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You can get the value of a constant by simply specifying its name.
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Unlike with variables, you should <emphasis>not</emphasis> prepend
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a constant with a <literal>$</literal>.
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You can also use the function <function>constant</function> to
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read a constant's value if you wish to obtain the constant's name
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dynamically.
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Use <function>get_defined_constants</function> to get a list of
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all defined constants.
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</simpara>
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<note>
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<simpara>
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Constants and (global) variables are in a different namespace.
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This implies that for example &true; and
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<varname>$TRUE</varname> are generally different.
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</simpara>
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</note>
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<simpara>
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If you use an undefined constant, PHP assumes that you mean
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the name of the constant itself, just as if you called it as
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a <type>string</type> (CONSTANT vs "CONSTANT"). An error of level
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<link linkend="ref.errorfunc">E_NOTICE</link> will be issued
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when this happens. See also the manual entry on why
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<link linkend="language.types.array.foo-bar">$foo[bar]</link> is
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wrong (unless you first <function>define</function>
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<literal>bar</literal> as a constant). If you simply want to check if a
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constant is set, use the <function>defined</function> function.
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</simpara>
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<para>
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These are the differences between constants and variables:
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<itemizedlist>
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<listitem>
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<simpara>
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Constants do not have a dollar sign (<literal>$</literal>)
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before them;
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Constants may only be defined using the
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<function>define</function> function, not by simple assignment;
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Constants may be defined and accessed anywhere without regard
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to variable scoping rules;
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Constants may not be redefined or undefined once they have been
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set; and
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</simpara>
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</listitem>
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<listitem>
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<simpara>
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Constants may only evaluate to scalar values.
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</simpara>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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<example>
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<title>Defining Constants</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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define("CONSTANT", "Hello world.");
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echo CONSTANT; // outputs "Hello world."
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echo Constant; // outputs "Constant" and issues a notice.
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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<example>
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<title>Defining Constants using the <literal>const</literal> keyword</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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// Works as of PHP 5.3.0
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const CONSTANT = 'Hello World';
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echo CONSTANT;
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<note>
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<para>
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As opposed to defining constants using <function>define</function>,
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constants defined using the <literal>const</literal> keyword must be
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declared at the top-level scope because they are defined at compile-time.
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This means that they cannot be declared inside functions, loops or
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<literal>if</literal> statements.
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</para>
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</note>
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<simpara>
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See also <link linkend="language.oop5.constants">Class Constants</link>.
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</simpara>
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</sect1>
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<sect1 xml:id="language.constants.predefined">
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<title>Magic constants</title>
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<simpara>
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PHP provides a large number of <link
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linkend="reserved.constants">predefined constants</link> to any script
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which it runs. Many of these constants, however, are created by
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various extensions, and will only be present when those extensions
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are available, either via dynamic loading or because they have
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been compiled in.
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</simpara>
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<para>
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There are eight magical constants that change depending on
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where they are used. For example, the value of
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<constant>__LINE__</constant> depends on the line that it's
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used on in your script. These special constants are
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case-insensitive and are as follows:
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</para>
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<para>
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<table>
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<title>A few "magical" PHP constants</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row xml:id="constant.line">
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<entry><constant>__LINE__</constant></entry>
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<entry>
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The current line number of the file.
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</entry>
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</row>
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<row xml:id="constant.file">
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<entry><constant>__FILE__</constant></entry>
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<entry>
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The full path and filename of the file. If used inside an include,
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the name of the included file is returned.
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Since PHP 4.0.2, <constant>__FILE__</constant> always contains an
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absolute path with symlinks resolved whereas in older versions it contained relative path
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under some circumstances.
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</entry>
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</row>
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<row xml:id="constant.dir">
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<entry><constant>__DIR__</constant></entry>
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<entry>
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The directory of the file. If used inside an include,
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the directory of the included file is returned. This is equivalent
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to <literal>dirname(__FILE__)</literal>. This directory name
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does not have a trailing slash unless it is the root directory.
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(Added in PHP 5.3.0.)
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</entry>
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</row>
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<row xml:id="constant.function">
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<entry><constant>__FUNCTION__</constant></entry>
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<entry>
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The function name. (Added in PHP 4.3.0) As of PHP 5 this constant
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returns the function name as it was declared (case-sensitive). In
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PHP 4 its value is always lowercased.
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</entry>
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</row>
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<row xml:id="constant.class">
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<entry><constant>__CLASS__</constant></entry>
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<entry>
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The class name. (Added in PHP 4.3.0) As of PHP 5 this constant
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returns the class name as it was declared (case-sensitive). In PHP
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4 its value is always lowercased. The class name includes the namespace
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it was declared in (e.g. <literal>Foo\Bar</literal>).
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Note that as of PHP 5.4 __CLASS__ works also in traits. When used
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in a trait method, __CLASS__ is the name of the class the trait
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is used in.
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</entry>
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</row>
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<row xml:id="constant.trait">
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<entry><constant>__TRAIT__</constant></entry>
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<entry>
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The trait name. (Added in PHP 5.4.0) As of PHP 5.4 this constant
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returns the trait as it was declared (case-sensitive). The trait name includes the namespace
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it was declared in (e.g. <literal>Foo\Bar</literal>).
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</entry>
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</row>
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<row xml:id="constant.method">
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<entry><constant>__METHOD__</constant></entry>
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<entry>
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The class method name. (Added in PHP 5.0.0) The method name is
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returned as it was declared (case-sensitive).
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</entry>
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</row>
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<row xml:id="constant.namespace">
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<entry><constant>__NAMESPACE__</constant></entry>
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<entry>
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The name of the current namespace (case-sensitive). This constant
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is defined in compile-time (Added in PHP 5.3.0).
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</para>
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<para>
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See also
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<function>get_class</function>,
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<function>get_object_vars</function>,
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<function>file_exists</function>&listendand;
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<function>function_exists</function>.
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</para>
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</sect1>
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</chapter>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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indent-tabs-mode:nil
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sgml-parent-document:nil
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sgml-default-dtd-file:"~/.phpdoc/manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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